fair -
With respect to weather, generally descriptive of pleasant weather conditions, with due regard for location and time of year. It is subject to popular misinterpretation, for it is a purely subjective description. When this term is used in weather forecasts (National Weather Service), it is meant to imply 1) no precipitation; 2) less than 0.4 sky cover of low clouds; 3) no other extreme conditions of cloudiness, visibility, or wind; 4) unrestricted visibility; and 5) light winds of generally less than 10 knots (5 m s-1). Compare cloudy.

fall -
Same as autumn.

feed -
(Or antenna feed.) The source of illumination for an antenna reflector.

feedback -
A sequence of interactions that determines the response of a system to an initial perturbation. Feedbacks may either amplify (positive feedback) or reduce (negative feedback) the ultimate state of the system.

feeding -
(Rare.) Descriptive of a windstorm of increasing intensity.

fetch -
1. The distance upstream of a measurement site, receptor site, or region of meteorological interest, that is relatively uniform. If a measurement site is located in the middle of a farm field with homogeneous land use, and if there are no changes to the land use and no obstructions such as trees or buildings immediately upstream of the site, then the site is said to have "large fetch". Large fetch is usually considered good if the measurements are to be representative of the atmosphere over the farm field. Similarly, measurements over a homogeneous forest could also have large fetch if there are no clearcuts or changes in the tree characteristics upstream of the measurement site. 2. (Also called generating area.) An ocean area where waves are generated by a wind having a constant direction and speed. 3. The length of the fetch area, measured in the direction of the wind in which ocean waves are generated. In many cases, the fetch is limited by the upwind distance to the coast.

field -
In its restricted physical sense, any physical quantity that varies in three-dimensional space (and possibly time), usually continuously except possibly on surfaces or curves. Field quantities often satisfy partial differential equations. An example of a scalar field is the temperature T(x, y, z, t) at time t at each point (x, y, z) of a solid body; an example of a vector field is the (local) velocity fieldv(x, y, z, t) in a fluid, the separate parts of which are in motion relative to each other. The continuity of these fields is a mathematical fiction, obtained by averaging over volumes containing many atoms or molecules but still small on a macroscopic scale.

filling -
An increase in the central pressure of a pressure system on a constant-height chart, or an analogous increase in height on a constant-pressure chart; the opposite of deepening. The term is commonly applied to a low rather than to a high. Since filling is almost always accompanied by a decrease in the intensity of cyclonic circulation, it is frequently used to imply the process of cyclolysis. Filling can be defined quantitatively in at least two ways: 1) as the time rate of central-pressure increase; or 2) as that component of the pressuretendency that cannot be attributed to either the motion of the pressure system relative to that point or to the influence of atmospheric tides.

fix -
A geographical position determined by visual reference to the surface, by reference to one or more radio NAVAIDs, by celestial plotting, or by another navigational device.

flare -
A bright, transient event within the sun's chromosphere and corona. Flares produce enhanced emission at radio frequencies and in the ultraviolet and x-ray spectral regions as well. They may also produce increased particle emission, often with ions of cosmic ray energies. Flares usually appear within minutes and fade within an hour. They are localized to small areas (typically <10-3 of the solar disk) and usually occur within solar active regions.

flux -
1. The rate of flow of some quantity, often used in reference to the flow of some form of energy. See also power. 2. In the field of atmospheric turbulence and boundary layers, often used as a contraction for flux density; namely, the flow of a quantity per unit area per unit time. These fluxes can be defined in two forms: dynamic and kinematic. The dynamic flux of a quantity is the flow of that quantity per unit area per unit time, where often the word dynamic is assumed if it is not explicitly stated. The advantage of a kinematic flux is that it has units that are more easily measured by a conventional meteorological instrument. The units are usually a velocity (m s-1) times a temperature (K), specifichumidity (kgwater/kgair), or wind speed (m s-1).

fog -
Water droplets suspended in the atmosphere in the vicinity the earth's surface that affect visibility. According to international definition, fog reduces visibility below 1 km (0.62 miles). Fog differs from cloud only in that the base of fog is at the earth's surface while clouds are above the surface. When composed of ice crystals, it is termed ice fog. Visibility reduction in fog depends on concentration of cloud condensation nuclei and the resulting distribution of droplet sizes. Patchy fog may also occur, particularly where air of different temperature and moisture content is interacting, which sometimes make these definitions difficult to apply in practice. Fogs of all types originate when the temperature and dewpoint of the air become identical (or nearly so). This may occur through cooling of the air to a little beyond its dewpoint (producing advection fog, radiationfog or upslope fog), or by adding moisture and thereby elevating the dewpoint (producing steamfog or frontal fog). Fog seldom forms when the dewpoint spread is greater than 4°F. According to U.S. weather observing practice, fog that hides less than 0.6 of the sky is called ground fog. If fog is so shallow that it is not an obstruction to vision at a height of 6 ft above the surface, it is called simply shallow fog. In aviation weather observations fog is encoded F, and ground fog GF. Fog is easily distinguished from haze by its higher relative humidity (near 100%, having physiologically appreciable dampness) and gray color. Haze does not contain activated droplets larger than the critical size according to Köhler theory. Mist may be considered an intermediate between fog and haze; its particles are smaller (a few m[&mgr;]m maximum) in size, it has lower relative humidity than fog, and does not obstruct visibility to the same extent. There is no distinct line, however, between any of these categories. Near industrial areas, fog is often mixed with smoke, and this combination has been known as smog. However, fog droplets are usually absent in photochemical smog, which only contains unactivated haze droplets.

forerunner -
The low-frequency ocean swell that commonly arrives at the coast before the primary swell generated by a distant storm.

free atmosphere -
(Sometimes called free air.) That portion of the earth's atmosphere, above the planetary boundary layer, in which the effect of the earth's surface friction on the air motion is negligible, and in which the air is usually treated (dynamically) as an ideal fluid. The base of the free atmosphere is usually taken as the geostrophic wind level.

freeze -
1. See freezing. 2. The condition that exists when, over a widespread area, the surface temperature of the air remains below freezing (0°C) for a sufficient time to constitute the characteristic feature of the weather. This is a general term, and the timeperiod necessary is usually considered to be two or more days; only the hardiest herbaceous crops survive. It differs from a dry freeze or black frost, for these terms are usually used to describe purely local freezing due to chilling of the surface air by rapid radiation from a restricted portion of the earth. Compare light freeze, hard freeze, killing freeze.

freezing -
1. The phase transition of a substance passing from the liquid to the solid state; solidification; the opposite of fusion. In meteorology, this almost invariably applies to the freezing of water. The phase change from the gaseous to the solid state is deposition. Like condensation, the freezing of water involves the process of nucleation. See ice point, freezing point, true freezing point, melting point. 2. Said of an environment when its temperature is equal to or less than 0°C (32°F). See freeze.

freezing level -
Commonly, and in aviation terminology, the lowest altitude in the atmosphere, over a given location, at which the air temperature is 0°C; the height of the 0°C constant-temperature surface. This simple concept may become slightly complicated by the existence of one or more "above-freezing layers" formed by temperature inversions at altitudes higher than the above-defined freezing level. In cloud physics terminology, this is more accurately termed the melting level, for melting of ice always occurs very near 0°C, but liquid cloud drops may remain supercooled to much colder temperatures. See icing level.

freezing rain -
Rain that falls in liquid form but freezes upon impact to form a coating of glaze upon the ground and on exposed objects. In aviation weather observations, this hydrometeor is encoded ZR. While the temperature of the ground surface and glazed objects is typically near or below freezing (0°C or 32°F), it is necessary that the water drops be supercooled before striking. Freezing rain can sometimes occur on surfaces exposed to the air (such as tree limbs) with air temperatures slightly above freezing in strong winds. Local evaporational cooling may result in freezing. Freezing rain frequently occurs, therefore, as a transient condition between the occurrence of rain and ice pellets (sleet). When encountered by an aircraft in flight, freezing rain can cause a dangerous accretion of clear icing.

frequency -
1. The rate of recurrence of any periodic phenomenon, often associated with waves of all kinds. Without qualification frequency often means temporal frequency, the rate of recurrence of a time-varying function, but could mean spatial frequency, the rate of recurrence of a space-varying function. Spatial frequency is the reciprocal of the repeat distance (sometimes the wavelength). The dimensions of (temporal) frequency are inverse time. A common unit for frequency is cycle per second, formerly abbreviated cps, but superseded by hertz, abbreviated as Hz. The symbol n[&ngr;] is often used for frequency but f is common in engineering. Period is inverse frequency. Related to frequency, and applied especially to sinusoidally varying quantities, is angular or circular frequency, often denoted by w[&ohgr;] = 2p[&pgr;]n[&ngr;], with units radians per unit time (e.g., radians per second). 2. In statistics, the number of times a specified event occurs in a given series of observations; for example, the number of rainy days observed at a particular station during a certain period of time. In many types of studies (hydrometeorological, especially) the reciprocal of frequency, the recurrence interval, is used.

fresh -
Descriptive of air that is stimulating and refreshing. See fresh breeze, fresh gale.

front -
1. In meteorology, generally, the interface or transition zone between two air masses of different density. Since the temperature distribution is the most important regulator of atmospheric density, a front almost invariably separates air masses of different temperature. Along with the basic density criterion and the common temperature criterion, many other features may distinguish a front, such as a pressure trough, a change in wind direction, a moisture discontinuity, and certain characteristic cloud and precipitation forms. The term front is used ambiguously for 1) frontal zone, the three-dimensional zone or layer of large horizontal density gradient, bounded by 2) frontal surfaces across which the horizontal densitygradient is discontinuous (frontal surface usually refers specifically to the warmer side of the frontal zone); and 3) surface front, the line of intersection of a frontal surface or frontal zone with the earth's surface or, less frequently, with a specified constant-pressure surface. Types of front include polar front, arctic front, cold front, warm front, and occluded front. See also anafront, katafront, intertropical front, secondary front, upper front. 2. See wave front.

frontal passage -
(Acronym fropa.) The passage of a front over a point on the earth's surface; or, the transit of an aircraft through a frontal zone.

frost -
1. The fuzzy layer of ice crystals on a cold object, such as a window or bridge, that forms by direct deposition of water vapor to solid ice. 2. The condition that exists when the temperature of the earth's surface and earthbound objects fall below freezing. Depending upon the actual values of ambient-air temperature, dewpoint, and the temperature attained by surface objects, frost may occur in a variety of forms. These include a general freeze, hoarfrost (or white frost), and dry freeze (or black frost). If a frostperiod is sufficiently severe to end the growing season (or delay its beginning), it is commonly referred to as a killing frost. See frost day, ground frost. 3. See frozen ground. 4. Same as hoarfrost. Compare rime.

frost day -
An observational day on which frost occurs; one of a family of climatic indicators (e.g., thunderstorm day, rain day). The definition is somewhat arbitrary, depending upon the accepted criteria for a frost observation. Thus, it may be 1) a day on which the minimumair temperature in the thermometer shelter falls below 0°C (32°F); 2) a day on which a deposit of white frost is observed on the ground; 3) in British usage, a day on which the minimum temperature at the level of the ground or on the tops of low, close-growing vegetation falls to -0.9°C (30.4°F) or below (also called a "day with ground frost"); and perhaps others. The present trend is to drop such terms in favor of something less ambiguous, such as "day with minimum temperature below 0°C (32°F)."